Method for manufacturing yarn for vessel stent

ABSTRACT

A stent for a vessel implanted in the vessel of the living body including a main body portion of the stent formed into a tube by a yarn formed of a biodegradable polymer exhibiting a shape memory function. The main body portion of the stent is shape-memorized to a size that can be inplanted in the vessel. The main body portion of the stent is implanted in the vessel of the living body as it is contracted in diameter by an external force, and is enlarged in diameter by being heated with the temperature of the living body. The main body portion of the stent is formed by winding a yarn formed of a biodegradable polymer in a tube form as the yarn is bent in a zigzag design. The main body portion of the stent is enlarged or contracted in diameter with the bends of the yarn as the displacing portions.

This application is a continuation of U.S. application Ser. No.09/530,986 now U.S. Pat. No. 6,500,204 which was the National Stage ofInternational Application No. PCT/JP99/04884, filed Sep. 8, 1999.

TECHNICAL FIELD

This invention relates to a stent for the vessel mounted in the vessel,such as blood vessel, lymphatic vessel, bile duct or urinary duct tomaintain a constant state in the lumen of the vessel.

BACKGROUND ART

Heretofore, if a stenosis portion has occurred in the vessel of a livingbody, in particular the blood vessel, such as artery, a balloon formingportion provided in the vicinity of the distal end of the ballooncatheter is inserted into this stenosis portion. This balloon formingportion is expanded to form a balloon to expand the stenosis portion ofthe blood vessel to improve the blood flow, by way of the transcutaneousblood vessel forming technique (PTA).

It has been clarified that, if the PTA is applied, stenosis tends to beproduced at a high probability in the once stenosis portion.

In order to prevent this restenosis, the current practice is to apply atubular stent in the site processed with the PTA. This stent is insertedinto the blood vessel in a diameter-contracted state and subsequentlyimplanted in the blood vessel as it is expanded in diameter to supportthe blood vessel from its inside to prevent restenosis from beingproduced in the blood vessel.

As this sort of the stent, there have so far been proposed a balloonexpanding stent and a self-expanding stent.

The balloon expanding stent is applied over a balloon provided in afolded and diameter-contracted state in a catheter and, after beinginserted in the targeted site for implantation, such as a site oflesion, where the blood vessel is stenosis, the balloon is expanded andincreased in diameter to support the inner surface of the blood vessel.Once expanded in diameter, the balloon expanding stent is fixed in thisexpanded state and cannot be deformed in keeping with the pulsations ofthe blood vessel wall. On the other hand, if the balloon expanding stentis deformed after being expanded in diameter and implanted in thiscondition in the blood vessel, it cannot be restored to its originalexpanded state, such that there is the risk that the stent cannotsupport the inner surface of the blood vessel reliably.

The self-expanding stent is housed in the diameter-contracted state in aholder, such as a tube, having an outer diameter smaller than the innerdiameter of the targeted site for implantation in the blood vessel, andis inserted in the targeted site for implantation in the blood vessel asit is housed in a holder. The stent, thus inserted in the targeted sitefor implantation in the blood vessel, is extruded or extracted from theholder so as to be expanded in diameter to the pre-contracted state, byexploiting the force of restoration proper to the stent, thus continuingto support the inner wall of the blood vessel.

As this sort of the self-expanding stent, there is proposed such a oneobtained on warping a linear member of metal, such as stainless steel,into a sinusoidal or zig-zag design, to form a tube.

With the self-expanding stent formed from a metal linear member, theouter diameter prevailing at the time of expansion is difficult tocontrol precisely, such that the stent is likely to be expandedexcessively in comparison with the inner diameter of the blood vessel inwhich it is implanted. Moreover, if the force of holding the stent inthe contracted state is removed, the stent is expanded abruptly. If thestent inserted into the blood vessel is expanded abruptly, the innerwall of the blood vessel is likely to be injured.

As the self-expanding stent, those formed of shape memory alloys, suchas T—Ni, Ti—Ni—Cu or Ti—Ni—Fe based alloys, have been proposed.

The stent, formed of shape memory alloys, is kept to its size when it isimplanted in the targeted loading site in the blood vessel, by the shapememory action, and is subsequently contracted in diameter, so as to beinserted in this diameter-contracted state in the blood vessel. Afterinsertion into the targeted loading site in the blood vessel, this stentis expanded to the size of the shape memory and subsequently exhibitssuper-elasticity under the body temperature of the living body tocontinue supporting the inner wall of the blood vessel.

Since the shape memory alloy has extremely high tenacity, such that itexerts an extremely large mechanical pressure to a portion of the innerwall of the blood vessel, thus possibly damaging the blood vessel.Moreover, there are occasions wherein the stent formed of a shape memoryalloy is not uniformly expanded in diameter against the inner wall ofthe blood vessel when implanted in the blood vessel. If a portion of thestent compresses against the inner wall of the blood vessel prematurelyto commence to be expanded in diameter, the blood vessel cannot beexpanded uniformly. In this case, the portion of the blood vessel,against which a portion of the stent has compressed prematurely, isenlarged excessively in diameter, and hence is likely to be damaged.

The stent formed of metal such as shape memory alloy, once implanted inthe vessel, such as blood vessel, is permanently left in the living bodyunless it is taken out by surgical operations.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a stent for avessel, such as blood vessel, which is able to keep the vessel in theexpanded state reliably without injuring the vessel.

It is another object of the present invention to provide a stent for avessel which disappears after lapse of a pre-set period afterimplantation in the vessel to eliminate the necessity of executing asurgical operation of taking out the stent from the vessel afterrestoration of the site of lesion.

It is another object of the present invention to provide a stent for avessel which is able to support the vessel, such as blood vessel, with auniform force.

It is yet another object of the present invention to provide a stent fora vessel which can be inserted into a meandering vessel, such as bloodvessel, with good trackability, and which can be easily and reliablyimplanted in the targeted site in the vessel.

For accomplishing the above object, the present invention provides astent for a vessel implanted in the vessel of the living body includinga main body portion of the stent formed into a tube by a yarn formed ofa biodegradable polymer exhibiting a shape memory function. The mainbody portion of the stent is shape-memorized to a size that can beretained in the vessel. The main body portion of the stent is implantedin the vessel of the living body as it is contracted in diameter by anexternal force, and is enlarged in diameter by being heated with thebody temperature of the living body.

The yarn used is a concatenated continuous monofilament yarn or amulti-filament yarn made up of a plurality of monofilament yarns unifiedtogether.

The main body portion of the stent is formed by the yarn formed of abiodegradable polymer being wound to a tube as the yarn is bent in azigzag design and is enlarged or contracted in diameter with the bendsof the yarn as displacing portions.

In the main body portion of the stent, at least part of neighbouringbends of the yarns wound to a tube as the yarns are bent in a zigzagdesign are connected to one another so that a pre-set tubular shape ofthe main body portion of the stent is positively maintained oncontracting or enlarging its diameter.

The tubular main body portion of the stent is formed by arraying pluralyarns each connected to form a ring as each yarn is bent in a zigzagdesign, these yarns being juxtaposed along the axial direction of themain body portion of the stent to form a tube.

Each yarn making up the main body portion of the stent is formed of abiodegradable polymer having the glass transition temperature lower thanapproximately 70° C. Thus, the main body portion of the stent isenlarged in diameter to its shape-memorized state at a temperature closeto the body temperature.

Each yarn making up the main body portion of the stent is formed of abiodegradable polymer compounded from one or more of polylactic acid(PLLA), polyglycolic acid (PGA), a copolymer of polyglycolic acid andpolylactic acid, polydioxanone, a copolymer of trimethylene carbonateand glycolid, and a copolymer of polyglycolic acid or polylactic acidand ε-caprolactone.

If a radiopaque medium is mixed into or deposited on the yarn, the stateof implantation of the stent in the vessel can be easily checked fromoutside the living body using X-rays.

If antithrombotic drugs or drugs for suppressing neointimal formationare mixed into or deposited on the yarn formed by the biodegradablepolymer, these drugs can be administered in a sustained fashion as thestent is dissolved.

Moreover, if a radiation source radiating β-rays or a radiation sourceradiating γ-rays is mixed into or deposited on the yarn formed of thebiodegradable polymer, these rays can be radiated to the lesion as thestent is inserted into the living body, thus assuring sustainedirradiation of radiation rays.

Other objects and advantages of the present invention will becomeapparent from the following description which is made with reference tothe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view showing a stent for the vessel according to thepresent invention.

FIG. 2 is a perspective view showing a yarn constituting the stentaccording to the present invention.

FIG. 3 is a perspective view showing another yarn constituting the stentaccording to the present invention.

FIG. 4 is a plan view showing the bent state of the yarn constituting amain body portion of the stent.

FIG. 5 is an enlarged plan view showing a portion of the main bodyportion of the stent.

FIG. 6 is a perspective view showing the state of how shape memory isafforded to the stent for the vessel.

FIG. 7 is a perspective view showing the state of diameter contractionof a stent for vessel in shape memory to the diameter expanded state.

FIG. 8 is a plan view showing the bent state of the yarn when the stentfor vessel is contracted in diameter.

FIG. 9 is a plan view of the stent for vessel showing itsdiameter-contracted state.

FIG. 10 is a graph showing temperature characteristics of the stent forvessel according to the present invention.

FIG. 11 is a perspective view showing another embodiment of the stentfor vessel according to the present invention.

FIG. 12 is a side view showing the state in which the stent for vesselaccording to the present invention is inserted into the blood vessel.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring to the drawings, a stent 1 for the vessel according to thepresent invention is explained in detail.

The stent 1 for the vessel according to the present invention is used asit is inserted into the blood vessel such as coronary artery of a livingbody and includes a tubular main body portion 3 of the stent comprisedof a yarn 2 of a biodegradable polymer having the shape memory function,as shown in FIG. 1.

The yarn 2 is formed of a biodegradable polymer which does not affectthe living body when the yarn is implanted in a living body, such as ahuman body. As this biodegradable polymer, polylactic acid (PLLA),polyglicolic acid (PGA), polyglactin (copolymer of polyglycolic acid andpolylactic acid), polydioxanone, polygliconate (copolymer oftrimethylene carbonate and glicolid), or a copolymer of polyglicolicacid or polylactic acid and ε-csaprolactone. It is also possible to usea biodegradable polymer obtained on compounding two or more of thesematerials.

The yarn 2 of the biodegradable polymer may be formed using a screwextruder. For forming the yarn 2 using the screw extruder, pelletsformed of a biodegradable polymer as a starting material are heated at atemperature lower than the melting point Tm and dried in vacua. Thepellets are charged into a hopper of the screw extruder and melted undercompression and heating to a temperature in the vicinity of the meltingpoint Tm or a temperature higher than the melting point and lower thanthe thermal decomposition point. This melted biodegradable polymer isextruded from a nozzle set at a temperature lower than the melting pointTm and higher than the glass transition temperature (Tg). This extrudedbiodegradable polymer is rolled up to form a linear member which then isfurther drawn to form the yarn 2 employed in the present invention.

The yarn 2 thus formed is a monofilament comprised of a concatenation ofthe biodegradable polymer, as shown in FIG. 2.

The yarn 2 employed in the present invention may not only be themonofilament but a multifilament yarn comprised of plural monofilaments2 a, as shown in FIG. 3.

The yarn 2 formed by the aforementioned screw extruder using thebiodegradable polymer as explained above, is composed of cross-linkedpolymer molecules and exhibits shape memory properties.

The yarn 2 employed in the present invention may not only be of acircular cross-section but also of a flat cross-section.

The yarn 2, formed as explained above, is bent in a zig-zag design inconcatenated vee shapes and wound spirally to constitute a tubular mainbody portion of the stent 3 as shown in FIG. 4. A spirally wound shapeof the yarn 2 is obtained with a side of a bend 4 of the vee shape as ashort portion 4 a and with its opposite side as a long portion 4 b. Bysetting the lengths of the short portion 4 a and the long portion 4 bbetween the bends 4 so as to be approximately equal to each other, theapices of the neighbouring bends 4 are contacted with each other, asshown in FIG. 5. Part or all of the apices of the contacted bends 4 arebonded to one another. The yarn 2 of the main body portion of the stent3 is positively maintained in the state of keeping the tubular shape bybonding the apices of the bends 4 contacting with each other.

The bends 4 having the apices contacting with each other are bondedtogether by melting and fusing the contact portions together on heatingthe contact portions to a temperature higher than the melting point Tm.

The stent 1, constituted using the tubular main body portion of thestent 3, is shape-memorized to the size with which it is implanted inthe blood vessel. For realizing this shape memory, the stent 1 isequipped on a shaft-like mold frame 101 sized to maintain the size ofthe stent 1 implanted in the vessel of the living body, and is heated toa temperature higher than the glass transition temperature (Tg) andlower than the melting point of the biodegradable polymer constitutingthe yarn 2, so as to be deformed to a size consistent with the size ofthe mold frame 101. The stent 1 equipped on the mold frame 101 then iscooled, along with the mold frame 101, to a temperature lower than theglass transition temperature (Tg). This affords to the stent 1 the shapememory properties so that the stent is fixed in the deformed state.

The heating for deforming the stent 1 to afford shape memory thereto isachieved by a heating oven.

The stent 1, obtained in this manner, is shape-memorized to the diameterR1 of approximately 3 to 5 mm and to the length L1 of 10 to 15 mm, asshown in FIG. 1. This size corresponds to or is larger than the diameterwith which the stent is implanted in the blood vessel of the livingbody.

The stent 1 equipped and shape-memorized on the mold frame 101 iscontracted in diameter after it is dismounted from the mold frame 101.This contraction in diameter occurs as the main body portion of thestent 3 is deformed under a mechanical force applied from the outerperimeter of the main body portion of the stent 3 in the state in whichthe stent is cooled to a temperature lower than the glass transitiontemperature (Tg). The diameter contraction of the stent 1 is realized bythrusting the main body portion of the stent 3 into adiameter-contracting groove 202 provided in a diameter-contracting moldframe 201 as shown in FIG. 7. This diameter-contracting groove 202 isformed as a recessed groove in the major surface of thediameter-contracting mold frame 201 to permit facilitated insertion ofthe elongated stent 1.

The stent 1, thus pushed into the inside of the diameter-contractinggroove 202, is contracted in diameter by displacing the bends 4 so thatthe opening angle θ1 of the bend 4 will be a smaller opening angle θ2,as shown in FIG. 8. This diameter contraction, achieved by displacingthe bends 4, is by deforming the bends 4 of the yarn 2 cooled to atemperature lower than the glass transition temperature (Tg). Forexample, in the stent 1, shape-memorized to the diameter R1 ofapproximately 3 to 5 mm, the diameter is reduced to a diameter R2 ofapproximately 1 to 2 mm, as shown in FIG. 9.

By this diameter contraction, the stent 1, shape-memorized to thediameter-expanded state, is slightly elongated in the longitudinaldirection from the shape-memorized state.

The stent 1, pushed into the diameter-contracting groove 202 provided inthe diameter-contracting mold frame 201, and thereby contracted indiameter, is pulled out from an opened end 203 of thediameter-contracting groove 202. The stent 1, produced from the yarn 2formed of the biodegradable polymer, is kept after dismounting from thediameter-contracting mold frame 201 at a temperature lower than theglass transition temperature (Tg) to maintain the strain afforded to thebends 4 representing the displacement portions to keep thediameter-contracted state.

For contracting the diameter of the stent 1, shape-memorized to thediameter-enlarged state, it is possible to use a variety of differentmethods other than the above-described method of employing thediameter-contracting mold frame 201. For example, the stent 1 may becontracted in diameter by applying a mechanical force from the outerperimeter of the shape-memorized stent 1 without using mold frames.

If the stent 1, contracted in diameter by application of an externalforce, is heated to a temperature higher than the glass transitiontemperature (Tg), it is relieved of the strain afforded to the bends 4,so that the bend 4 folded to the small opening angle θ2 is opened to theopening angle θ1 to restore to its original shape-memorized size. Thatis, the stent 1 on being re-heated to a temperature higher than theglass transition temperature (Tg) is enlarged to its originalshape-memorized size, as shown in FIG. 1.

Meanwhile, the stent 1 for the vessel, according to the presentinvention, is used as it is inserted into the blood vessel, such as thecoronary vessel of the living body, and is enlarged in diameter to theshape-memorized state, when inserted into the blood vessel, to supportits inner wall. It is noted that the yarn 2, making up the main bodyportion of the stent 3 of the stent 1 for the vessel, is formed of abiodegradable polymer, with the glass transition temperature (Tg) lowerthan 70° C., in order to restore to its original shape by thetemperature equal or close to body temperature of the living body.

The stent 1, formed by the yarn 2, which has the glass transitiontemperature (Tg) lower than 70° C. and which is able to restore to itsoriginal shape by the body temperature of the living body, can be heatedat a temperature not producing heat damages to the blood vessel of theliving body, even if it is heated for enlarging its diameter to itsshape-memorized state.

The stent 1, implanted on the blood vessel in the diameter-contractedstate, is enlarged in diameter to realize the size capable of contactingwith the inner wall of the blood vessel by a balloon provided on acatheter. On diameter expansion into contact with inner wall of theblood vessel by the balloon, the stent 1 can be evenly contacted withthe inner wall of the blood vessel and heated evenly by the bodytemperature to restore to its original shape.

If the heated contrast medium is injected into the balloon through acatheter to restore the stent 1 to its original shape, the heatingtemperature of approximately 50° C. suffices, thus not producing heatdamages to the blood vessel.

The temperature dependency in shape restoration of the stent 1 formed bythe yarn 2 of polylactic acid (PLLA) with the glass transitiontemperature (Tg) of approximately 57° C., and the stent 1 formed by theyarn 2 of polyglycolic acid (PGA) with the glass transition temperature(Tg) of approximately 37° C. was indicated.

The yarn 2 was produced as a drawn monofilament, with a diameter of 50to 300 μm, using the above-described screw extruder, from polylacticacid (PLLA) and polyglycolic acid (PGA). Using this yarn 2, each stent 1is formed by bending in a zigzag design as explained above and is woundto a tube with a diameter R1 of 4 mm by shape memory action. The tubethus produced was then contracted to the diameter R2 of 1.4 mm. Eachstent 1 in the shape-memorized state is of a length L1 of 12 mm.

The stent 1, formed by the yarn 2 of polylactic acid PLLA, restores toits original shape at 70° C. in only 0.2 sec, as shown at A in FIG. 10,while recovering its shape at 50° C. in 13 sec and moderately recoveringits shape at 37° C. close to the body temperature over approximately 20minutes. At 20° C. or less, close to the room temperature, the stent 1is kept in the diameter-contracted state without recovering the shape.

Thus, with the stent 1, formed from the yarn 2 of polylactic acid PLLA,the time needed in shape restoration can be controlled by controllingthe heating temperature. Therefore, the rate of shape restoration can becontrolled in keeping with the state of the blood vessel in which isimplanted the stent 1.

On the other hand, the stent 1, formed from the yarn 2 of polyglycolicacid (PGA), restores to its original shape at 45° C. in only 0.5 second,as shown at B in FIG. 10, while restoring to its original shape in abouta second at 37° C. close to the body temperature and in 10 seconds at30° C. lower than the body temperature. At 15° C. or less, close to roomtemperature, the diameter-contracted state is maintained without shaperecovery.

The stent 1 formed by the yarn 2 of polyglycolic acid (PGA), having alow glass transition temperature (Tg), restores to its original shaperapidly by body temperature on insertion into the blood vessel. Thus,the stent 1 can be applied with advantage to such application in whichthe stent needs to be enlarged in diameter as soon as it is insertedinto the blood vessel. Moreover, since the stent can recover to itsoriginal shape promptly with the body temperature without heating, heatcontrol for shape restoration of the stent 1 is facilitated.

In the stent for vessel 1, described above, the sole yarn 2, bent in azigzag design for forming bends partway, is wound spirally to form atubular main body portion of the stent 3. Alternatively, a sole yarn,bent in a zigzag design for forming bends partway, may be formed into aring, and a plurality of these yarns 21, wound into rings, may then bearrayed side-by-side along the axial direction to form a tubular mainbody portion of the stent 23, as shown in FIG. 11.

With this main body portion of the stent 23, the apex portions of thebends 24 of the respective juxtaposed yarns 21, contacting with eachother, are bonded together to maintain the tubular shape reliably.

The stent 1, comprised of the main body portion of the stent 23, isequipped on the shaft-like mold frame 101, as in the case of the stent 1described above. The stent 1 of the present embodiment is again heatedto a temperature higher than the glass transition temperature (Tg) ofthe biodegradable polymer constituting the yarn 21 and lower than themelting point Tm, and is shape-memorized to a size with which the stentwas implanted in the vessel of the living body. The stent then iscontracted to a diameter by e.g., a diameter-contracting mold frame 201,which will allow the stent to be easily introduced into the vessel ofthe living body.

It suffices if the stent 1 of the present invention is formed as theyarn 2 is bent in a zigzag design to a tube. A variety of methods may beused for winding the yarn in this manner.

Meanwhile, the shape memory restoring force of the shape memory alloyused in a conventionally proposed stent is roughly tens of kilograms(kg)/mm², whereas that of the biodegradable polymer constituting theyarn of the stent according to the present invention is roughly severalkg/mm². That is, the biodegradable polymer having the shape memoryfunction has a shape memory restoring rest which is appreciably lowerthan that of the shape memory alloy. Moreover, the rate of recovery tothe shape-memorized state of the biodegradable polymer having the shapememory function can be ten times that of the shape memory alloy. Thestent formed using the yarn of the biodegradable polymer having theshape memory function having these characteristics can be restored toits original shape memorized state in a time interval not less than 10times for the stent formed of the shape memory alloy.

Thus, the stent formed of the yarn of the biodegradable polymer havingsuch characteristics that the shape memory restoring force is small andthe time of restoration to the shape memorized state is long, isenlarged in diameter evenly without abrupt increase in diameter, if thestent in the contracted-diameter state is inserted into the blood vesseland subsequently enlarged in diameter. Moreover, there is no risk ofexcessive mechanical pressure being applied to the inner wall of theblood vessel, thus positively preventing the possibility of damaging theblood vessel.

On the other hand, the yarn formed of the biodegradable polymer havingthe shape memory function has a coefficient of friction smaller thanthat of the linear member formed of metal, such as shape memory alloy,so that, if the stent is abutted against a portion of the inner wall ofthe blood vessel during the time the stent is increased in diameter, itslips and expands uniformly on the inner wall surface of the bloodvessel without inflicting damages to the blood vessel.

It has been clinically demonstrated that, although a stent used forpreventing restenosis of the blood vessel retains its shape for severalweeks to several months after it is implanted in the blood vessel, itdesirably disappears in several months after implantation.

Since the stent according to the present invention is formed by the yarnof a biodegradable polymer, it retains its shape for several weeks toseveral months after it is implanted in the blood vessel of a livingbody, however, it is absorbed into the living tissue to vanish inseveral months after it is implanted in the blood vessel.

A variety of drugs may be mixed into the yarn of the polymer fibers. Ifradiopaque agent is mixed at the time of spinning the yarn, the statusof the stent for the vessel can be observed with X-rays, so thatthrombolytic drug or antithrombotic drug, such as heparin, urokinase ort-PA may be mixed into the yarn to prevent thrombotic restenosis of theblood vessel. Moreover, drugs can be continuously administered. If aradiation source radiating β- or γ-rays is mixed into or coated on theyarn, the lesion site in the living body can be illuminated by theradiations in a sustained and concentrated fashion.

Moreover, by admixing drugs aimed at suppressing the neo intimalhyperplasia on the yarn, it is possible to administer drugs aimed atsuppressing the neointimal formation in a sustained fashion.

It is noted that the radiopaque agent, thrombolytic drug orantithrombotic drug, pharmaceuticals aimed at suppressing the neointimalformation, or the radiation source, may also be coated on the surface ofthe spun yarn.

The stent 1 according to the present invention is constituted by windingthe biodegradable polymer yarns, having the shape memory function, in atube without overlapping, while it can be flexed and deformed easily inthe longitudinally, as shown in FIG. 12, and hence can be inserted withgood trackability into a bent blood vessel 301, because the stent 1 isformed by winding the yarns of the biodegradable polymer having theshape memory function into a tube without the yarns overlapping with oneanother. In particular, the stent 1, formed using a yarn having bendspartway, can be easily deformed in the longitudinal direction and hencecan be introduced into the bent blood vessel 301 with high trackability.

On the other hand, the stent 1 of the present invention is formedwithout producing overlapping portions of the yarns 2, and can bedisplaced in the shape-memorized state with the bends 4 of the yarns 2as the displacing portions. Therefore, the stent 1 can restore its shapesmoothly without encountering the resistance by the overlapped yarns 2.

In addition, in the stent 1 of the present invention, in which the yarns2 are wound without forming overlapping portions, there is no superposedyarn to reduce the damages otherwise inflicted to the wall of the bloodvessel.

INDUSTRIAL APPLICABILITY

Since the stent for vessel according to the present invention isconstituted using a biodegradable polymer having the shape memoryfunction, the stent can memorize its shape to a size with which it isimplanted in the vessel, so that the vessel can be positively maintainedin the expanded state without being damaged.

Also, the stent can be easily enlarged in diameter after it is implantedin the vessel, such as blood vessel, and also can support the vessel,such as blood vessel, with an even force, so that there may be provideda stent for vessel that is able to hold the vessel in a stabilized statein a reliably diameter-enlarged state.

In particular, since the stent for vessel according to the presentinvention is formed using a biodegradable polymer, it can retain itsshape for several weeks to several months after it is implanted in theblood vessel, however, the stent can vanish in several months after itis implanted. Thus, the stent may be provided which is clinically mostdesirable.

1. A method for manufacturing a yarn for a vessel stent comprising thesteps of: compressing and melting pellets formed of a biodegradablepolymer, heating said pellets to a temperature in the vicinity of itsmelting point (Tm) or a temperature higher than its melting point andlower than its thermal decomposition point by a screw extruder,extruding said melted biodegradable polymer from a nozzle of said screwextruder to form a yarn, and bending said yarn in a zig zag design toconstitute a tubular main body portion of said vessel stent to obtainsaid yarn for the vessel stent, said nozzle being controlled to have atemperature not higher than the melting point and not lower than theglass transition temperature of the biodegradable polymer.
 2. A methodfor manufacturing a yarn for a vessel stent according to claim 1 whereinsaid nozzle is controlled to have the temperature higher than the glasstransition temperature (Tg) of said biodegradable polymer.
 3. A methodfor manufacturing a yarn for a vessel stent according to claim 1 whereinsaid pellets formed of the biodegradable polymer are heated at atemperature lower than its melting point (Tm) under drying in vacuum andthen charged into the screw extruder.
 4. A method for manufacturing ayarn for a vessel stent according to claim 1 wherein said yarn extrudedfrom the screw extruder is drawn further.
 5. A method for manufacturinga yarn for a vessel stent according to claim 1 wherein said yarnextruded from the screw extruder is formed into a non-interruptedcontinuous monofilament.
 6. A method for manufacturing a yarn for avessel stent according to claim 1 wherein said biodegradable polymer hasthe glass transition temperature (Tg) lower than approximately 70° C. 7.A method for manufacturing a yarn for a vessel stent according to claim1 wherein said biodegradable polymers are one or more from amongpolylactic acid (PLLA), polyglycolic acid (PGA), a copolymer ofpolyglycolic acid and polylactic acid, polydioxanone, a copolymer oftrimethylene carbonate and glycollide, and a copolymer of polyglycolicacid or polylactic acid and ε-caprolactone.
 8. A method formanufacturing a yarn for a vessel stent according to claim 1 whereinsaid biodegradable polymer matrix contains one or more of an X-raynon-transmitting agent, an anti-thrombotic agent, pharmaceuticals forsuppressing neointima hyperplasia, a β-ray radiation source and a γ-rayradiation source.
 9. A method for manufacturing a yarn for a vesselstent according to claim 1 further comprising a step of depositing oneor more of an X-ray non-transmitting agent, an anti-thrombotic agent,pharmaceuticals for suppressing neointima hyperplasia, a β-ray radiationsource and a γ-ray radiation source on the surface of said yarn.
 10. Amethod for manufacturing a yarn for a vessel stent comprising the stepsof: (A) obtaining a yarn made by the following process: (1) compressingand melting pellets formed of a biodegradable polymer, (2) heating saidpellets to a temperature in the vicinity of its melting point (Tm) or atemperature higher than its melting point and lower than its thermaldecomposition point by a screw extruder, and (3) extruding said meltedbiodegradable polymer from a nozzle of said screw extruder to form saidyarn, and (B) bending said yarn in a zig zag design to constitute atubular main body portion of said vessel stent to obtain said yarn for avessel stent, said nozzle being controlled to have a temperature nothigher than the melting point and not lower than the glass transitiontemperature of the biodegradable polymer.
 11. A method for manufacturinga yarn for a vessel stent according to claim 10 wherein said nozzle iscontrolled to have the temperature higher than the glass transitiontemperature (Tg) of said biodegradable polymer.
 12. A method formanufacturing a yarn for a vessel stent according to claim 10 whereinsaid pellets formed of the biodegradable polymer are heated at atemperature lower than its melting point (Tm) under drying in vacuum andthen charged into the screw extruder.
 13. A method for manufacturing ayarn for a vessel stent according to claim 10 wherein said yarn extrudedfrom the screw extruder is drawn further.
 14. A method for manufacturinga yarn for a vessel stent according to claim 10 wherein said yarnextruded from the screw extruder is formed into a non-interruptedcontinuous monofilament.
 15. A method for manufacturing a yarn for avessel stent according to claim 10 wherein said biodegradable polymerhas the glass transition temperature (Tg) lower than approximately 70°C.
 16. A method for manufacturing a yarn for a vessel stent according toclaim 10 wherein said biodegradable polymers are one or more from amongpolylactic acid (PLLA), polyglycolic acid (PGA), a copolymer ofpolyglycolic acid and polylactic acid, polydioxanone, a copolymer oftrimethylene carbonate and glycollide, and a copolymer of polyglycolicacid or polylactic acid and ε-caprolactone.
 17. A method formanufacturing a yarn for a vessel stent according to claim 10 whereinsaid biodegradable polymer matrix contains one or more of an X-raynon-transmitting agent, an anti-thrombotic agent, pharmaceuticals forsuppressing neointima hyperplasia, a β-ray radiation source and a γ-rayradiation source.
 18. A method for manufacturing a yarn for a vesselstent according to claim 10 further comprising a step of depositing oneor more of an X-ray non-transmitting agent, an anti-thrombotic agent,pharmaceuticals for suppressing neointima hyperplasia, a β-ray radiationsource and a γ-ray radiation source on the surface of said yarn.